The development of novel techniques to study processes and phenomena at the nanoscale is of fundamental importance for nanoscience and nanotechnology. Only by fully understanding nanoscale processes and the physics and materials science of nanoparticles and nanostructures, can nanotechnological solutions be applied widely, with appropriate risk management. The heart of the anticipated technological advances through “nanotechnology” is the wide range of novel physical and chemical properties of nanoparticles and nanostructures. The typical size range of nanoparticles/nanostructures used to define nanotechnology is 1—a few hundred nanometers (nm), the exact upper boundary depending on the property studied and the application in mind. These novel exciting properties of nanoparticle systems can be advantageous in the development of both novel sensors and scientific tools to characterize and learn more about the physics and materials science of nanoparticle systems, as well as in developing new technologies for applications in biotechnology, medicine, clean tech, engineering, etc.
An inherent feature of metallic nanoparticles is a collective coherent oscillation of the conduction electrons—the localized surface plasmon resonance (LSPR), which can be excited by external photons. In other words, the LSPR is a possible excited state of the metal nanoparticle electron system, which can be excited by photons or, equivalent^, by the electromagnetic field of light incident on the particle. The LSPR excitation is a consequence of the inter-electronic (collective) interactions of the electrons combined with spatial confinement of the conduction band electron system within the nanoparticle volume. An electron density wave is formed with a frequency/wavelength/energy that depends on the electronic structure of the nanoparticle, its geometry, size and dielectric environment.
The spectral sensitivity of the LSPR, that is the amount of spectral shift of the LSPR along the wavelength/frequency/energy axis (alternatively, a spectral sensitivity of a LSPR is also measurable in terms of a change of the peak height, the peak full-width-at-half-maximum (FWHM) or also as a change in the optical extinction or transmission spectrally close to the LSPR frequency), to events taking place on the surface of and close to the nanoparticles opens up the possibility to use “plasmonic” nanoparticles as transducers in sensors in general, where applications as biosensors have been most exploited so far. “Events” in the preceding sentence are e.g. refractive index changes in the surrounding medium induced by adsorption of biomolecules onto the nanoparticle surface. Most of these reported applications of LSPR in plasmonic biosensors rely on the sensitivity of the nanoparticle LSPR to the dielectric constant of the surrounding medium, opening up a route to “refractive index sensing” where adsorbate-induced changes in the local dielectric environment are utilized for detection of e.g. molecular binding events on the nanoparticle surface and in the particle nano-environment. In a typical nanoplasmonic refractive index sensor the events to be detected take place directly on the surface of e.g. plasmonically active gold nanoparticles. This is motivated by the strong enhancement of the electric field in the close (<ca. 50 nm, depending on particle size and material and geometry) proximity of the nanoparticle surface. During the actual sensing, a change of the local refractive index at the interface between nanoparticle and surrounding medium, induced by e.g. adsorption or chemical binding of molecules from solution to the sensor nanoparticle surface, is detected as a change of the particle's optical response.
In addition, nano-LSPR sensors open up the possibility for miniaturization because of the small size of the transducers i.e. the plasmonic nanoparticles. Another related advantage is the small detection volume making it possible to measure extremely small amounts of analytes. Due to their small size, nanoscale sensors offer the potential for multiplexing (i.e. making sensor arrays where each sensor has a different sensitivity). One application of multiplexing is fingerprinting, where a large number of sensors with slightly different sensitivities, are used in a sensor array. Even if the specificity and selectivity of each individual sensor may be low and not very informative alone, the combined response from all sensors can, using pattern recognition, provide a highly accurate and informative response.
Many different nanoparticle shapes have been investigated for their potential use in plasmonic refractive index (bio-) sensors, including disks, triangles, rods, ellipses, wires, spheres, cubes, stars, holes in a thin metal film, nanoshells and core-shell particles, nanorice and nanorings.
Plasmonic refractive index sensing platforms comprising a nanoparticle based sensing structure, exhibiting and relying on a confined optical excitation like LSPR are previously known.
As an alternative to plasmonic refractive index sensing it has also been shown that by using a “direct sensing” approach one is able to measure an induced structural (e.g. size and/or geometry) change and a change of the electronic structure of a plasmonic nanoparticle upon absorption of atoms into the plasmonic nanoparticle. In particular, this approach has been demonstrated for measurements of hydrogen uptake in Pd nanoparticles. Thus, during a direct sensing event, the sensor nanoparticles themselves are affected/changed by the process to be sensed. The latter process may ultimately lead to altered physical properties of the nanoparticles and give rise to a (measured) change of their optical response, i.e. their LSPR. Typically, the event to be sensed induces a spectral shift and/or a change of the spectral linewidth and/or the optical cross-section of the measured optical excitation of the nanoparticle. This can be detected as a significant altering of the optical transmission and/or extinction and/or absorption and/or scattering and/or reflection signature of the nanoparticle.